Fig. 2. Gastrocnemius muscle performance in a human (A) and guinea fowl (B) during running. Traces are scaled to align the stance periods of the running stride (broken lines). EMG traces in A and B are rectified and averaged over many stride cycles. (A) Average human gastrocnemius activity (EMG) relative to ground reaction force (Dietz et al., 1979), along with gastrocnemius fascicle length measured from ultrasound recordings [traced from fig. 3 in Lichtwark and Wilson (Lichtwark and Wilson, 2006)]. (B) Average guinea fowl gastrocnemus activity (EMG), muscle–tendon force and fascicle length [thin lines indicate s.e.m. (Daley, 2006)]. Note that the muscle is activated with similar timing and undergoes a similar strain pattern during stance in both the human and guinea fowl (as well as other animals). The muscle is activated in anticipation of stance, with increases in activity during stance suggestive of reflex feedback (e.g. Dietz et al., 1979).